Describe the bug
While AbstractPreAuthenticatedProcessingFilter::getPreAuthenticatedPrincipal is correctly annotated with @Nullable, the overriden method in RequestHeaderAuthenticationFilter is missing the annotation resulting in compilation errors when using Kotlin

To Reproduce
Create a Kotlin class that extends RequestHeaderAuthenticationFilter and override getPreAuthenticatedPrincipal.
Kotlin insists on non-nullable types, making null checks and null return values impossible.

Expected behavior
The return type should be correctly annotated with @Nullable, allowing null checks in Kotlin code overriding this method.

Sample

class EntraCompatibleRequestHeaderAuthenticationFilter : RequestHeaderAuthenticationFilter() {

    override fun getPreAuthenticatedPrincipal(request: HttpServletRequest): String {
        return super.getPreAuthenticatedPrincipal(request).toString().substringBefore('@')
    }

}

but should be

class EntraCompatibleRequestHeaderAuthenticationFilter : RequestHeaderAuthenticationFilter() {

    override fun getPreAuthenticatedPrincipal(request: HttpServletRequest): String? {
        return super.getPreAuthenticatedPrincipal(request)?.toString()?.substringBefore('@')
    }

}